OpenBSD

"OpenBSD can exist as it does today because of the example set by the Computer Systems Research Group at Berkeley and the battles which they and others fought to create a relatively un-encumbered Unix source distribution."

Early in the development of the Unix family of operating systems, the desire to develop software that could be freely used and improved was one of the major developments of the BSD operating system in reaction to restrictions on then available Unix operating system. To this day, many TCP/IP software stacks are built on the rock-solid code originally created for BSD.

As FOSS licenses go, BSD is perhaps the most radical, just short of public domain, requiring no more than keeping the license with the code that states basically that this is the only requirement.

OpenBSD is among the most popular of the BSD variants, and is particularly known for its strong security. OpenBSD supports binary emulation of most software from SVR4 (Solaris), FreeBSD, Linux, BSD/OS, SunOS, and HP-UX. The operating system is developed by volunteers, and development is funded by CDs, t-shirts, posters, and donations of time and cash from organizations and individuals.